The purpose of radiation therapy is tumor control. In view of clonogen proliferation it makes sense to deliver the necessary dose in as short a time as possible. The limiting factor in tumor treatment is normal tissue reaction: normal tissue reactions must not exceed "tolerance" level. The development and application of high technology particularly in computers and computer-based and assisted imaging has stimulated great progress in tumor localization and treatment planning, and even in the technology of delivery and its quality assurance, i.e., the spatial aspects of the issue. Decision-making as to the amount of dose and its distribution over &w (by fractionation or protraction) is still essentially empirical, however. The present project continues the development and exploration of a theoretical description of time-dose response of tumors as well as normal tissues. Its basic concepts have been published in 1988[1]. Since the, further developments have concentrated on extension of the Linear-Quadratic model. This has resulted in some further publications. The extension concerns a unified description of the influence of incomplete repair and comprises a description of the influence of tumor proliferation and stem cell/transition cell repopulation. Currently a study is being completed regarding the implications of high dose arate vs. low dose rate brachytherapy. This work includes the development of an interactive computer program to search for high dose rates comparable with constant tumor control as well as no more severe toxicity.